Sublimator Having A Porous Plate With Integral Primary And Secondary Heat Transfer Surfaces

ABSTRACT

A sublimator includes a porous plate having a first surface comprising a low pressure side and a second surface comprising a high pressure side such that refrigerant is configured to move through the porous plate from the high pressure side to the low pressure side. The second surface defines a primary heat transfer surface. The porous plate further includes a plurality of secondary heat transfer surfaces integrally formed on the primary heat transfer surface to facilitate flow and evenly distribute refrigerant across the high pressure side of the porous plate.

BACKGROUND OF THE INVENTION

The present invention generally relates to a sublimator and a method ofmaking a sublimator, and more specifically to a sublimator having aporous plate with integrally formed primary and secondary heat transfersurfaces.

A sublimator takes a fluid to be cooled and transfers the heat containedtherein to a refrigerant that is sublimated. Traditionally, sublimatorsinclude a porous plate through which the refrigerant sublimates and aplurality of fins that are individually attached to the porous plate.Depending upon the application, there can be several thousand fins thatare attached to the porous plate with each fin segment beingincrementally spot welded to the porous plate in many locations. Thespot welding is labor intensive and time consuming. Further, maintainingweld quality through this process requires constant tool maintenance andfrequent quality inspections which increases overall manufactory cost.

SUMMARY OF THE INVENTION

According to one exemplary embodiment, a sublimator includes a porousplate having a first surface comprising a low pressure side and a secondsurface comprising a high pressure side such that refrigerant isconfigured to move through the porous plate from the high pressure sideto the low pressure side. The second surface defines a primary heattransfer surface. The porous plate further includes a plurality ofsecondary heat transfer surfaces integrally formed on the primary heattransfer surface to facilitate flow and evenly distribute refrigerantacross the high pressure side of the porous plate.

In a further embodiment of the above, the plurality of secondary heattransfer surfaces comprise a plurality of discrete features that arenon-coplanar with the primary heat transfer surface and which are placedin a predetermined arrangement to optimize heat sink with heat fluxinput.

In another exemplary embodiment, a sublimator includes a refrigerantchamber having a first side and a second side, and further includes afluid chamber positioned on the first side of the refrigerant chamber,wherein the fluid chamber is configured to receive a fluid to be cooled.A porous plate has a first surface comprising a low pressure side and asecond surface comprising a high pressure side, wherein the highpressure side is positioned on the second side of the refrigerantchamber such that refrigerant is configured to move through the porousplate from the high pressure side to the low pressure side. The secondsurface of the porous plate defines a primary heat transfer surface, andthe porous plate includes a plurality of secondary heat transfersurfaces integrally formed on the primary heat transfer surface tofacilitate flow and evenly distribute refrigerant across the highpressure side of the porous plate.

In a further embodiment of any of the above, the sublimator includes aninlet to direct refrigerant into the refrigerant chamber to flow acrossthe primary and secondary heat transfer surfaces, and further includes arefrigerant supply in fluid communication with the inlet to replenishrefrigerant that sublimates from the low pressure side of the porousplate into an external environment.

In another exemplary embodiment, a method of making a sublimatorincludes providing a porous plate having a first surface comprising alow pressure side and a second surface comprising a high pressure sidesuch that refrigerant is configured to move through the porous platefrom the high pressure side to the low pressure side, and wherein thesecond surface defines a primary heat transfer surface, and integrallyforming a plurality of secondary heat transfer surfaces on the primaryheat transfer surface to facilitate flow and evenly distributerefrigerant across the high pressure side of the porous plate.

In a further embodiment of any of the above, the method includes usingan additive manufacturing process to integrally form the plurality ofsecondary heat transfer surfaces on the primary heat transfer surface.

These and other features of the present invention can be best understoodfrom the following specification and drawings, the following of which isa brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of a sublimator with a porousplate incorporating the subject invention.

FIG. 2 is a perspective view of the porous plate as used in thesublimator of FIG. 1.

FIG. 3 is a magnified view of a portion of the porous plate of FIG. 2.

DETAILED DESCRIPTION

FIG. 1 shows a sublimator 10 that includes a refrigerant chamber 12having a first side 14 and a second side 16. A fluid chamber 18 ispositioned on the first side 14 of the refrigerant chamber 12. The fluidchamber 18 is configured to receive a fluid 20 to be cooled. A porousplate 22 has a first surface 24 comprising a low pressure side Lp and asecond surface 26 comprising a high pressure side Hp. The high pressureside Hp is positioned on the second side 16 of the refrigerant chamber12 such that refrigerant 28 is configured to move through the porousplate 22 from the high pressure side Hp to the low pressure side Lp. Thesecond surface 26 of the porous plate 22 defines a primary heat transfersurface. The porous plate 22 also includes a plurality of secondary heattransfer surfaces 30 integrally formed on the primary heat transfersurface to facilitate flow and evenly distribute refrigerant across thehigh pressure side Hp of the porous plate 22.

The fluid chamber 18 comprises an area that is enclosed by a housing 32that includes an intermediate plate portion 34 that is located betweenthe refrigerant chamber 12 and the fluid chamber 18. While a singlefluid chamber 18 and a single refrigerant chamber are shown, it shouldbe understood that there could be additional fluid chambers 18 andadditional refrigerant chambers 12. This will be discussed in greaterdetail below.

The sublimator 10 includes an inlet 40 to direct the refrigerant 28 intothe refrigerant chamber 12 to flow across the primary and secondary heattransfer surfaces. A refrigerant supply 42 is in fluid communicationwith the inlet 40 to replenish refrigerant that sublimates from the lowpressure side Lp of the porous plate 22 into an external environment E,such as outer space for example. A header 44 fluidly connects the inlet40 to the refrigerant chamber 12.

As shown in FIGS. 2-3, the plurality of secondary heat transfer surfaces30 comprise a plurality of discrete features 46 that are non-coplanarwith the primary heat transfer surface. Further, the discrete features46 are placed in a predetermined arrangement to optimize heat sink withheat flux input. The discrete features 46 are integrally formed asone-piece with the porous plate 22 using an additive manufacturingprocess. This will be discussed in greater detail below. In one example,the discrete features 46 comprise a plurality of discrete protrusions orfins extending outwardly from the primary heat transfer surface. Thesefeatures/fins are formed from a perforated or porous material such thatthe refrigerant can flow through the features in the manner describedbelow.

The sublimator 10 is used with a refrigerant 28 that has a triple pointwhere equilibrium of vapor, liquid, and solid will occur at apredetermined temperature or pressure and there is available anenvironment at or below this condition. In one example, the refrigerant28 comprises water; however, other types of refrigerant could also beused. The refrigerant 28 is directed into the refrigerant chamber 12from the pressurized supply 42. The refrigerant 28 then passes throughthe porous material that forms the porous plate 22 and freezes whenexposed to the low pressure side Lp to form a layer of ice 50 thatblocks further refrigerant 28 from exiting the low pressure side Lp ofthe porous plate 22.

The refrigerant 28 sublimates into the external environment E as heat isconducted to the porous plate 22 due to the heat exchange between thefluid 20 to be cooled and the refrigerant 28 in the refrigerant chamber12. As the refrigerant 28 sublimates away from the porous plate 22 andthe solid refrigerant becomes depleted, more refrigerant isautomatically used to replenish the porous plate 22.

In one example, the porous plate 22 is comprised of a stainless steelmaterial having a pore size of approximately 0.5 microns. Other types ofporous materials could also be used; however, the material needs to havea porous characteristic that facilitates formation of the necessarylayer of ice 50 for sublimation. Each pore essentially becomes pluggedwith ice that has a surface exposed to the outer space environment E. Assublimation occurs at this surface, the thickness of the layer of ice 50is reduced until it can no longer support the internal pressure withinthe chamber 12 and the refrigerant will begin to pass into the externalenvironment E. When the refrigerant is exposed to this lower pressurelevel below its triple point, the refrigerant freezes and reforms theice.

In the example shown, the entire high pressure side Hp of the porousplate 22 is overlaid on the refrigerant chamber 12 to provide maximumexposure. The discrete features 46 formed on the porous plate furtherenhance flow and improve distribution across and through the porousplate 22. This allows the formation of a uniform sheet of ice 50 acrossthe low pressure side Lp of the porous plate 22. The fluid 20 that is tobe cooled transmits heat through the intermediate plate portion 34 andthrough the refrigerant 28 and eventually into the porous plate 22. Theheat sublimates the ice at a rate that is directly proportional to theheat load and the fluid 20 to be cooled is discharged at a temperaturethat is lower than when the fluid entered the fluid chamber 18.

As discussed above, while only a single fluid chamber or passage 18 isshown in FIG. 1, the sublimator 10 can include multiple fluid passagesin parallel. Adjacent to each fluid passage 18 is a refrigerant passageor chamber 12, which are fed by a common inlet header 44. Therefrigerant flows in via the header 44 and exits the sublimator eitherby sublimation or evaporation depending on the “sink” temperature. The“sink” is the porous plate 22, and as the range of heat flux into theporous plate 22 varies, the heat rejection capability of the sublimatormoves coincidently. This relationship drives the demand for efficientintegral construction of the refrigerant passage or chamber 12 withrespect to the primary (porous plate surface) and secondary(protrusions/fins) heat transfer surfaces. Proper layout of thesecondary heat transfer surfaces is essential to balancing the heat fluxto heat sink temperature relationship. Maintaining a tight control bandon the sink capacity will produce a fleet of sublimators with minimalunit to unit variability in heat rejection capability.

In current configurations, fins are spot welded to the porous plate in afins per inch (FPI) arrangement to allow the sublimating refrigerant tooverlay the entire surface of the porous plate which is subjected to anatmosphere whose pressure will cause the refrigerant to freeze. Varyingthe FPI allows control of the transmitted heat flux at sensitivelocations on the porous plate. Spot welding perforated fin stock to theporous plate is a manufacturing method which leads to variability in theoverall heat transfer effectiveness of a sublimator. Spot welding is aprocess with inherent quality instability. The process is laborintensive, time consuming and requires constant tool maintenance to staywithin the required quality tolerance. Spot welding is used to provide athermal and structural connection between primary and secondary heattransfer surfaces. The variability of quality in the small spot weldseffects the ability of heat to transfer from the fluid to be cooled tothe refrigerant within the porous plate. This poor connection equates toa larger device for a given heat load.

Within a given refrigerant passage, the heat flux will vary over theporous plate surface due to the flow arrangement of the passage of thefluid to be cooled. A large heat flux in a localized area will result ina failure of the ice layer, which impedes the self-regulating nature ofthe sublimation process within the sublimator and allows carry-over ofthe refrigerant. The FPI of a porous plate can be varied to reduce theeffectiveness of a region on the surface of the porous plate to preventthis breakdown of the sublimation process. However, spot welding thefins to the plate to achieve the variable FPI increases labor costs.

The subject invention provides a sublimator 10 with a single part thathas integral primary and secondary heat transfer features. Thiseliminates the thermo-mechanically joined plate and fin configuration. Avariety of additive manufacturing methods can be used to produce theintegrally formed primary and secondary heat transfer surfaces. Theprocess is used to form or grow the discrete features 46 directly on theporous plate 22. Processes such as laser-sintering, stereolithography,and fused deposition modeling are just some of the example processesthat could be used to integrally form the features on the porous plate.The shape, size, location and density of the features can be varied asneeded to produce the optimum heat sink characteristics to preciselymatch the heat flux input.

By eliminating spot welding and tailoring the shape, size, location anddensity of the features to increase the heat transfer efficiency, thesize and weight of the sublimator can be reduced. Further, performanceof the sublimator becomes more consistent with a lighter and higherperforming unit in less volume. Also, labor and manufacturing costs aresignificantly decreased as manufacturing complexity is reduced andquality assurance verification procedures are not required as frequentlycompared to prior designs.

The preceding description is exemplary rather than limiting in nature.Variations and modifications to the disclosed examples may becomeapparent to those skilled in the art that do not necessarily depart fromthe essence of this disclosure. The scope of legal protection given tothis disclosure can only be determined by studying the following claims.

Although a combination of features is shown in the illustrated examples,not all of them need to be combined to realize the benefits of variousembodiments of this disclosure. In other words, a system designedaccording to an embodiment of this disclosure will not necessarilyinclude all of the features shown in any one of the Figures or all ofthe portions schematically shown in the Figures. Moreover, selectedfeatures of one example embodiment may be combined with selectedfeatures of other example embodiments.

1. A sublimator comprising: a porous plate having a first surfacecomprising a low pressure side and a second surface comprising a highpressure side such that refrigerant is configured to move through theporous plate from the high pressure side to the low pressure side, andwherein the second surface defines a primary heat transfer surface; anda plurality of secondary heat transfer surfaces integrally formed on theprimary heat transfer surface to facilitate flow and evenly distributerefrigerant across the high pressure side of the porous plate.
 2. Thesublimator according to claim 1 wherein the plurality of secondary heattransfer surfaces comprise a plurality of discrete protrusions extendingoutwardly from the primary heat transfer surface.
 3. The sublimatoraccording to claim 2 including an intermediate plate having a first sideand a second side facing opposite the first side, wherein the first sideis spaced apart from the high pressure side of the porous plate todefine a refrigerant chamber, and wherein the second side at leastpartially encloses a fluid chamber configured to cool a fluid within thefluid chamber.
 4. The sublimator according to claim 3 including an inletto direct refrigerant into the refrigerant chamber to flow across theprimary and secondary heat transfer surfaces.
 5. The sublimatoraccording to claim 4 including a refrigerant supply in fluidcommunication with the inlet to replenish refrigerant that sublimatesfrom the low pressure side of the porous plate into an externalenvironment.
 6. The sublimator according to claim 1 wherein theplurality of secondary heat transfer surfaces comprise a plurality ofdiscrete features that are non-coplanar with the primary heat transfersurface and which are placed in a predetermined arrangement to optimizeheat sink with heat flux input.
 7. A sublimator comprising: arefrigerant chamber having a first side and a second side; a fluidchamber positioned on the first side of the refrigerant chamber, whereinthe fluid chamber is configured to receive a fluid to be cooled; and aporous plate having a first surface comprising a low pressure side and asecond surface comprising a high pressure side, wherein the highpressure side is positioned on the second side of the refrigerantchamber such that refrigerant is configured to move through the porousplate from the high pressure side to the low pressure side, and whereinthe second surface of the porous plate defines a primary heat transfersurface, and wherein the porous plate includes a plurality of secondaryheat transfer surfaces integrally formed on the primary heat transfersurface to facilitate flow and evenly distribute refrigerant across thehigh pressure side of the porous plate.
 8. The sublimator according toclaim 7 wherein the plurality of secondary heat transfer surfacescomprise a plurality of discrete features that are non-coplanar with theprimary heat transfer surface and which are placed in a predeterminedarrangement to optimize heat sink with heat flux input.
 9. Thesublimator according to claim 7 wherein the plurality of secondary heattransfer surfaces comprise a plurality of discrete protrusions extendingoutwardly from the primary heat transfer surface.
 10. The sublimatoraccording to claim 7 including an inlet to direct refrigerant into therefrigerant chamber to flow across the primary and secondary heattransfer surfaces, and a refrigerant supply in fluid communication withthe inlet to replenish refrigerant that sublimates from the low pressureside of the porous plate into an external environment.
 11. Thesublimator according to claim 10 including a header fluidly connectingthe inlet to the refrigerant chamber.
 12. A method of making asublimator comprising the steps of: providing a porous plate having afirst surface comprising a low pressure side and a second surfacecomprising a high pressure side such that refrigerant is configured tomove through the porous plate from the high pressure side to the lowpressure side, and wherein the second surface defines a primary heattransfer surface; and integrally forming a plurality of secondary heattransfer surfaces on the primary heat transfer surface to facilitateflow and evenly distribute refrigerant across the high pressure side ofthe porous plate.
 13. The method according to claim 12 including usingan additive manufacturing process to integrally form the plurality ofsecondary heat transfer surfaces on the primary heat transfer surface.14. The method according to claim 13 including forming the plurality ofsecondary heat transfer surfaces as a plurality of discrete featuresthat are non-coplanar with the primary heat transfer surface andpositioning the plurality of discrete features in a predeterminedarrangement to optimize heat sink with heat flux input.
 15. The methodaccording to claim 14 including forming the plurality of discretefeatures as a plurality of protrusions extending outwardly from theprimary heat transfer surface.